The disclosure relates to glass-ceramics and precursor glasses that are crystallizable to glass-ceramics, which may each or both be strengthened by ion exchange processes; methods for making the same and articles including the same. In particular, this disclosure relates to translucent or opaque glass-ceramics that include a total crystalline phase up to about 20% by weight and crystal phases such as anatase, rutile, armalcolite, or a combination thereof, and the precursor glasses used to form such glass-ceramics.
In the past decade, electronic devices such as notebook computers, personal digital assistants, portable navigation device, media players, mobile phones, portable inventory devices and other such devices (frequently referred to as “portable computing devices”) have converged, while at the same time becoming small, light, and functionally more powerful. One factor contributing to the development and availability of such smaller devices is an ability to increase computational density and operating speed by ever decreasing electronic component sizes. However, the trend to smaller, lighter, and functionally more powerful electronic devices presents a continuing challenge regarding design of some components of the portable computing devices.
Components associated with portable computing devices encountering particular design challenges include the enclosure or housing used to house the various internal/electronic components. This design challenge generally arises from two conflicting design goals—the desirability of making the enclosure or housing lighter and thinner, and the desirability of making the enclosure or housing stronger and more rigid. Lighter enclosures or housings, typically thin plastic structures with few fasteners, tend to be more flexible while having a tendency to buckle and bow as opposed to stronger and more rigid enclosures or housings, typically thicker plastic structures with more fasteners having more weight. Unfortunately, the increased weight of the stronger, more rigid plastic structures might lead to user dissatisfaction, while the bowing and buckling of the lighter structures might damage the internal/electronic components of the portable computing devices, which almost certainly can lead to user dissatisfaction. Furthermore, plastics are easily scratched due to their low hardness, so their appearance degrades with use.
Among known classes of materials are glass-ceramics that are used widely in various other applications. For example, glass-ceramics are used widely in kitchens as cooktops, cookware, and eating utensils, such as bowls, dinner plates, and the like. Transparent glass-ceramics are used in the production of oven and/or furnace windows, optical elements, mirror substrates, and the like. Glass-ceramics are typically made by crystallizing precursor glasses, which are formulated to be crystallizable, at specified temperatures for specified periods of time to nucleate and grow crystalline phases in a glass matrix.
In some instances, it is desirable to form glass-ceramic articles for use in portable computing devices having specific optical properties, such as opacity and color. Known glass-ceramics based on the SiO2—Al2O3—Li2O glass system include those having either β-quartz solid solution (“β-quartz ss” or “β-quartz”) as the predominant crystalline phase or β-spodumene solid solution (“β-spodumene ss” or “β-spodumene”) as the predominant crystalline phase. Such known glass-ceramics can require specific heat treatment conditions to achieve a desirable color and exhibit lower strength, which may be due to the size and shape of the crystals, their stress filed in the glass and the strength of the residual glass. In addition, such glass ceramics may also exhibit an undesirable level of brittleness, which may be due to a high concentration of crystalline phase(s). Furthermore such glass-ceramics tend to have liquidus viscosities that preclude the use of high throughput forming methods such as float, slot draw, or fusion draw. For example, known glass-ceramics are formed from precursor glasses having liquidus viscosities of about 10 kP, which are not suitable for fusion draw, where liquidus viscosities of above 100 kP or above 200 kP are generally required. Accordingly, although glass-ceramics exhibit desirable properties such as high opacity, various degrees of translucency, and surface luster, which are generally not achievable by fusion forming, such glass-ceramics cannot take advantage of the pristine surfaces and thinness (e.g., 2 mm or less) achieved by fusion forming process.
In view of the foregoing problems with existing enclosures or housings, there exists a need for glass-ceramic and precursor glass materials that are ion exchangeable and with high liquidus viscosities (i.e. liquidus viscosities that enable forming methods such as slot draw, fusion draw, and the like), which provide improved enclosures or housings for portable computing devices, in a potentially more cost effective manner. Also, there exists a need for such materials that provide improved color properties (e.g., whiteness levels) and/or other opaque colors while addressing in an aesthetically pleasing manner the design challenges of creating lightweight, strong, and rigid enclosures or housings.